Parable of the parabola

Abstract

We study triangles and quadrilaterals which are inscribed in a circle and circumscribed about a parabola. Although these are particular cases of the celebrated Poncelet's Theorem, in this paper we do not assume the theorem, but prove it along the way. Similarly, our arguments are logically independent from the Cayley condition—describing points of a finite order on an elliptic curve or any other use of the theory of elliptic curves and functions. Instead, we use purely planimetric methods, including the Joachimsthal notation, to fully describe such polygons and associated circles and parabolas. We prove that a circle contains the focus of a parabola if and only if there is a triangle inscribed in the circle and circumscribed about the parabola. We prove that if the center of a circle coincides with the focus of a parabola, then there exists a quadrilateral inscribed in the circle and circumscribed about the parabola. We further prove that the quadrilaterals obtained in such a way are antiparallelograms. If the center of a circle does not coincide with the focus of a parabola, then a quadrilateral inscribed in the circle and circumscribed about the parabola exists if and only if the directrix of the parabola contains the point of intersection of the polar of the focus with respect to the circle with the line determined by the center and the focus. That point coincides with the intersection of the diagonals of any quadrilateral inscribed in the circle and circumscribed about the parabola. In particular, for a given circle and a confocal pencil of parabolas with the focus different from the center of the circle, there is a unique parabola for which there exists a quadrilateral circumscribed about it and inscribed in the circle.